What is the benefit of forcing developers to provide access to old games that require online functionality indefinitely, instead of just hard limiting them to say 10 years wich is essentially indefinite in terms of non-live service games.

In a choice between "you can play online until 2035" and "you can play online forever", the answer is pretty obvious. All things being equal, the indefinite option is better. I think the problem is that all things are not equal, and making it a legal requirement that all games with online features come with a guarantee those features work indefinitely is incredibly vague and can lead to situations that outright hurt developers.

If the devs need to provide a server binary for players to host a server, how do they ensure these servers only allow players who have purchased the game to play? If they can't ensure it, then the law is forcing companies to allow pirate servers to exist

How do they ensure people running these community servers aren't charging money for people to play? If they can't ensure it, then the law is allowing people to use a company's IP to generate money without a licence.

If the original version had an in-game shop where you can unlock things with real life money but the offline version doesn't have a shop, thus making parts of the game forever unobtainable, did they follow the law? If not, then devs would have to give out paid features for free.

Unless these kinds of details are accounted for, this vague idea is doomed to fail because no government is going to force a company to give up their copyright/IP for free. I know a lot of people have also said "fuck these giant corporations" but this also affects indie developers as well. Copyright protects small creators as much as it does large ones.